Wood-resin composite products, such as plywood, laminated veneer lumber (LVL), oriented strand lumber (OSL), oriented strand board (OSB), particleboard, medium density fiber board, hardboard and the like, traditionally have been made by combining a binder resin with wood components to form a stack or mat which is then consolidated in a hot platen press to cure the binder resin. The hot pressing process generally relies on a combination of conductive and convective heat flow emanating from the press platens to provide the necessary heat to cure the resin. The transfer of heat to the wood-resin composite by conductive and convective heat flow becomes increasing less effective as the thickness of the wood-resin composite being manufactured increases.
In an effort to produce thicker wood-resin composites more effectively, especially from wood pieces such as wood stands, chips, fibers or flakes, manufacturers have turned to hot presses that use saturated steam as the primary heat transfer medium. When saturated steam is introduced directly into the wood-resin composite (known as steam injection pressing), steam rapidly and directly heats the composite through its entire thickness, in part by some of the steam condensing on the pieces of wood and thus giving up its latent heat of condensation. Since the condensation and thus the heating takes place throughout the composite, resin curing occurs rapidly even with thick composites.
While the injection of steam into the wood composite during hot pressing greatly improves the rate of heating, the increased moisture load resulting from the condensed steam has limited the range of resin chemistries that have been successfully commercialized using this technology. In particular, polymethylene diphenyl diisocyanate binders (pMDIs) have been the mainstay for making wood composites by steam injection pressing, while condensation resins, and particularly phenol-formaldehyde resins, have yet to enjoy wide acceptance for this market. This is unfortunate as phenol-formaldehyde resins are known to provide cost-effective, strong, highly durable bonds with wood components made by conventional hot platen pressing techniques. Thus, the development of a way to use a phenol-formaldehyde binder resin successfully for making commercially acceptable wood composites by steam injection pressing would have significant value.